A) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device having semiconductor photodiodes as photoelectric conversion elements and insulated gate type transistors.
B) Description of the Related Art
As an image sensor, semiconductor image sensors are widely used which have semiconductor photodiodes as photoelectric conversion elements. There are two types of semiconductor image sensors: a CCD type semiconductor image sensor having charge coupled devices for detecting and transferring charges; and a MOS type image sensor having a charge detection circuit including MOS transistors for detecting charges. A transistor having a gate insulating film not made of only a single oxide film is also called herein MOS transistor. A MOS type semiconductor image sensor has an advantage that a consumption power can be reduced.
FIG. 1A is a schematic plan view showing the structure of a MOS type semiconductor image sensor. Pixels PX having photoreceptors are disposed in a pixel area in a matrix shape. In the structure shown, a transfer line TL, a reset line RL and a select line SL are disposed in parallel in a row direction, and a voltage line VRL and a signal read line SGL are disposed in parallel in a column direction. A vertical driver V-DRIVE drives each line, and a horizontal driver H-DRIVE reads signal voltage in each column. Signal voltage in pixels in each row are read, noises are cancelled in each column, and the signal voltage are amplified, A/D converted and subjected to other necessary processing.
FIG. 1B shows an example of the structure of a pixel PX. This example shows a 4Tr active pixel sensor (APS) in which one pixel is constituted of one photodiode PD and four MOS transistors. Anodes of photodiodes of all pixels are made of a common p-type well. A cathode (n-type region) of the photodiode PD constitutes a charge accumulation region. The cathode accumulates electrons among pairs of electrons and holes generated by photoelectric conversion of incidence light in the photodiode PD.
A transfer transistor TRT has as its source the cathode of the photodiode and as its drain a floating diffusion FD which is an n-type region in an electrically floating state in a p-type region. A transfer gate TG of the transfer transistor TRT controls charge transfer between the source and drain. As a transfer signal is supplied from the transfer line TL to pixels PX in the same row at the timing when an image sensing period is terminated, signal charges accumulated in the photodiodes PD in the row are transferred to corresponding floating diffusions FD.
A source follower transistor SFT and a select transistor SLT are serially connected, and their gates are connected to the floating diffusion FD and the select line SL, respectively. The floating diffusion FD generates a voltage V=Q/C where Q is transferred charges and C is a capacitance, and supplies an output corresponding to the accumulated charges to the gate of the source follower transistor SFT.
When a select signal is supplied from the select line SL to pixels in the same row, an output signal from each source follower transistor SFT is supplied to the signal read line SGL via the select transistor SLT. The signal charges after its read operation is completed become unnecessary. When an on-signal is supplied from the reset line to each reset transistor RST in the same row, charges in the floating diffusion FD are drained to the voltage line VRL via the reset transistor RST.
FIG. 1C is a timing chart illustrating a pixel drive operation. This timing chart shows changes in the signal TG applied to the gate of the transfer transistor TRT, in a signal RST applied to the gate of the reset transistor RST, in a signal SL applied to the gate of the select transistor SLT, in an output signal SGL supplied from SLT to the output signal line SGL and in a voltage FD at the floating diffusion FD (at the gate of the source follower transistor SFT).
As the gate voltage of the select transistor is made high, the select transistor enters an on-state capable of transferring an output voltage of the source follower transistor SFT. The gate voltage of the reset transistor RST is once made high to drain noise charges in the floating diffusion FD and prepare for a signal read operation. As the voltage of RST changes, a change in the voltage of a positive polarity occurs at the floating diffusion FD and source follower transistor SFT, i.e., at the output voltage SGL.
TG is made high at the timing when the image sensing period is terminated and the transfer transistor TRT is turned on to transfer charges accumulated in the photodiode PD to the floating diffusion FD. A change in the potential of a positive polarity occurs also at the floating diffusion FD and signal read line SGL, because of a change in the TG voltage. After stabilization, a potential at the floating diffusion FD having the capacitance C changes by ΔV=Q/C because of the read charges Q. This potential change is converted into an output signal by the source follower transistor SFT, and the output signal is read to the read line SGL. Thereafter, the potential of the select line SL becomes low to terminate a read operation.
After pixel structures of a semiconductor image sensor are formed, an interlayer insulating film is formed on the upper surface of the semiconductor substrate and contact plugs connected to the electrodes of transistors are formed through the interlayer insulating films. In order to form contact plugs on electrodes of a transistor, it is necessary to first form contact holes through the interlayer insulating film. It is preferable to form an etch stopper such as a silicon nitride film under the interlayer insulating film, in order not to damage the semiconductor substrate. In order to reduce noises of a photodiode and improve an image quality, it is desired to lower an interfacial level. To this end, it is desired to execute hydrogenation treatment. A silicon nitride film has a function of shielding hydrogen during hydrogenation treatment. Therefore, if contact holes are to be formed properly, the image quality is hindered.
Japanese Patent Laid-open Publication No. 2004-165236 proposes to deposit a silicon nitride film as an etch stopper by low pressure (LP) CVD and remove the silicon nitride film in a region above photodiodes and the like where the silicon nitride film is not necessary. This Publication explains that since contact holes can be formed properly because of the existence of the etch stopper, and since hydrogen can reach the surface of a semiconductor substrate from the region where the etch stopper is removed, dark current can be suppressed by hydrogen treatment to improve the image quality.
A semiconductor image sensor is desired to have a high sensitivity and an output with small noises.